This invention relates to optical systems for measuring properties of a sample, e.g., the thickness of a thin film.
An optical measurement called four-wave mixing (FWM) has been used previously to measure a variety of different material properties, such as film thickness, and film delamination. In FWM measurements, two excitation laser beams are overlapped in time and space on a sample s surface to form a spatially varying optical interference pattern. The pattern consists of alternating "light" (i.e., constructive interference) and "dark" (i.e., destructive interference) regions; the spacing between these regions depends on the wavelength of the laser beams and the angle therebetween. In a class of FWM measurements called impulsive stimulated thermal scattering (ISTS), the excitation laser beam contains a series of short (e.g., a few hundred picoseconds) optical pulses. These pulses of radiation are absorbed by the sample in the light regions, but not in the dark regions, to excite a "transient grating". This process heats and thermally expands the irradiated regions to launch coherent, counter-propagating acoustic waves whose wavelength and direction match those of the interference pattern. When ISTS is used to measure strongly absorbing films (e.g., metal films), the acoustic waves generate a time-dependent "ripple" on the film's surface that oscillates at the acoustic frequency. A probe beam then diffracts off the transient grating to form a series of signal beams, each of which represents a different diffracted order (e.g, the .+-.1 and .+-.2 orders). The signal beams oscillate in intensity at the acoustic frequency. One of the signal beams is detected and monitored to measure the properties of the sample.
Use of ISTS to measure film thickness and a variety of other properties is described, for example, in U.S. Pat. 5,633,711 (entitled MEASUREMENT OF MATERIAL PROPERTIES WITH OPTICALLY INDUCED PHONONS), U.S. Ser. No. 08/377,308 (entitled OPTICAL MEASUREMENT OF STRESS IN THIN FILM SAMPLES, filed Jan. 24, 1995), U.S. Ser. No. 08/783,046 (entitled METHOD AND DEVICE FOR MEASURING FILM THICKNESS, filed Jul. 15, 1996), and U.S. Ser. No. 08/926,850 (entitled METHOD AND APPARATUS FOR MEASURING THE CONCENTRATION OF IONS IMPLANTED IN SEMICONDUCTING MATERIALS, filed concurrently herewith), the contents of which are incorporated herein by reference.
There are several optical systems that generate the two excitation beams needed for ISTS. In a typical measurement, for example, a single excitation beam passes through a beam-splitter to form two excitation beams of roughly equal intensity. An imaging system (e.g., a lens) then collects these beams and then spatially overlaps them on the sample to form an optical interference pattern that includes the light and dark regions described above.
While useful for many types of FWM experiments, beam-delivery systems that generate two excitation beams, such as those using beam-splitters, suffer drawbacks. For example, these systems often have a relatively long depth of focus, i.e., the beams are overlapped for a relatively long distance. This makes it difficult to position the sample in the exact image plane of the imaging system. Another disadvantage is that the diffraction efficiency of the transient grating formed by two excitation beams is often quite small (e.g., on the order of 10.sup.-4 -10.sup.-5). This means that the diffracted signal beam is often weak and difficult to measure. A weak signal beam, in turn, makes it difficult to precisely measure the acoustic frequency and any corresponding property (e.g., film thickness).